Introduction

In the previous chapter we introduced the theory of geometrical optics, a very simplistic analysis of the propagation of radiation describing only the lines that trace the radiation trajectories. In that analysis, the lines, or rays, were not subjected to the effects of diffraction or interference; with the exception of dispersion, color too had no influence on these trajectories. The absolute value of the speed of light had no bearing on the propagation; only its magnitude relative to the speed in free space had to be known, and even that parameter could not be derived directly and had to be retrieved from other sources. Similarly, parameters of the important effect of dispersion could not be derived directly. Attenuation by absorption was outside the scope of geometrical optics, as were other effects related to the nature of radiation such as polarization, coherence, and wavelength. These shortcomings of geometrical optics were to be expected. After all, such fundamental questions as how radiation is created or how it interacts with a particular medium were not asked. Without consideration of these questions, the nature of radiation and the details of its propagation cannot be fully understood.

Historically, the first studies attempting to understand the nature of light, and not merely its patterns of propagation, were made in the seventeenth century. At that time, visible light was the only known mode of radiation.